That's Keppler. He's all over Bonsai Nut. Google grostone. It's glass, like pumice, but not made by a volcano, (more control) Get the smallest size, soil aerator. I use it as substrate with milled sphagnum, and there needs to be a layer of something to retard evaporation, like moss. But mostly I use good old pumice. I'm sure Al will chime in, he's good at that.

I've used it once or twice. There is some sort of pH buffering agent that requires washing or soaking. Seems very moisture retentive from observation. Never used it with a refined tree, but with some rooted cuttings.

Khaimraj suggests that the Na20 / Ca0 / Si02 might need to be Titanized or you might get some free sodium and calcium compounds coming out of the glass matrix.

If cullet is used in pottery, it tends to set like cement in glazes when exposed to moisture.

So far the marbles have shown no real surface degradation, and the tamarinds, which have acidic compounds at the root sections, have shown no problems with sodium compounds seeping out.Ditto the 3 mm glass spheres.More info later with the passing of time [ x years ].Good DayAnthony

Our name in Germany was spelled Kapler. Pronounced Keppler, made my Great Grandfather change it at Ellis Island.

I still use the growstones, but as has been mentioned, they dry very quickly. Owen needn't worry about water retention as there is none. Of course my summers are around continual 105 daily with temps at midnight still around 103. I am sure water retention would be better in a more mild climate.

FWIW, pumice dries just as fast here, so a switch to that wouldn't matter. I have a better particle anyway which does retain plenty of moisture.

Just as an aside.. The pictures my dad has of his grandfather show him all thorugh his life wearing a mustache just like Hitler did. That style was very popular in the last half of the 19th century and the first half of the 20th. Then some jerk came along and ruined the look forever more. He was struck by lightning twice and survived and drug by plow horses across a 100 acre field. He settled in Revenna Ohio where my Dad grew up. Didn't have indoor plumbing until the fifties.

I've used it once or twice. There is some sort of pH buffering agent that requires washing or soaking. Seems very moisture retentive from observation. Never used it with a refined tree, but with some rooted cuttings.